PROJECT SUMMARY. The demonstration in the late `80s and early `90s that specific strains of herpes and rabies virus are transported transneuronally exclusively at synapses in a time- and direction-dependent fashion opened up a new era in experimental neuroanatomy. Virus transneuronal transport is uniquely able to reveal the cellular composition of multi-synaptic circuits. As a consequence, this technique is providing fundamental new insights into the functional architecture of sensory, motor, cognitive and affective circuits in the central nervous system. The Center for Neuroanatomy with Neurotropic Viruses (CNNV) was created in 2004 to provide the neuroscience community with access to the highly specialized reagents, training and facilities that are necessary to use neurotropic viruses as transneuronal tracers. We document in this application that the CNNV has become an essential national and international resource for an important segment of the neuroscience community. Users of the Center are funded by 15 different institutes of the NIH, by NSF and by private foundations and international research organizations. The Center has established a strong history of providing well-characterized reagents, training and guidance in the use of the method, and specialized biohazard facilities for performing experiments using neurotropic viruses. The Center has served as an essential technical and intellectual resource for those interested in using this novel method. The goal of the Resource Component of this competitive renewal is to continue these essential functions of the Center.